In the diagnosis of a body by ultrasonic techniques, e.g. in the examination of the human body, ultrasonic energy radiated at the body is beamed to an internal target. Typically, a diagnostic probe is used in which a single crystal or an array of ultrasonic transducers is positioned behind the probe wear plate. The external surface of the wear plate is placed substantially in contact with a limited area of the patient's skin and a coupling medium is interposed between them which may take the form of a gel deposited as a thin coating on the skin area. The purpose of the coupling gel is to enhance acoustic coupling between the wear plate surface and the skin.
When the probe and other associated power equipment are operating normally, the probe beams relatively low levels of ultrasonic energy to the target, e.g. on the order of 10 milliwatts/cm.sup.2 averaged over both space and time. Under unusual circumstances, e.g. as a result of faulty operation or mishandling of the equipment, a malfunction or failure of the equipment may occur without knowledge of the patient or of the diagnostician. If the malfunction is such as to raise the acoustic intensity of the energy beamed to the target, a potential for harm to the patient may exist through thermal damage to the body or through ultrasonic cavitation. Thus, it becomes desirable to monitor the acoustic intensity of the applied energy on a continuous basis during the diagnostic procedure. This is especially important where the diagnostician, though medically trained, lacks the necessary technical training to distinguish between normal probe operation and an equipment malfunction. The monitoring procedure must therefore be simple and direct and provide an indication of equipment malfunction whenever the acoustic intensity of the applied ultrasonic energy exceeds a predetermined safety limit.
The use of thermographic techniques, whereby a liquid crystal material is applied to the skin and the color of the material is observed to provide an indication of skin temperature, is well established as a diagnostic tool. For example, mixtures of cholesteric liquid crystals such as cholesteryl oleyl carbonate, cholesteryl nonanoate, and cholesteryl benzonate, as disclosed in U.S. Pat. No. 3,533,399, show selective reflection near body skin temperatures that are visible as color changes to the eye of an observer. Where the liquid crystal material consists of a cream, it may be placed directly on the skin. However, the ability to observe the change of color of the material may be enhanced under certain conditions if the skin is first darkened with a water soluble dye, or with carbon black, in order to reduce extraneous reflections. As disclosed in U.S. Pat. No. 3,619,254, the material may also be silk screen printed onto the skin, e.g. by painting the skin through a silk screen, in order to control the thickness and uniformity of the coating so applied. An acrylic layer may be applied on top of the liquid crystal coating material for protection against degradation by the ambient air or by exposure to light. There also exist liquid crystal tapes which adhere directly to the skin.
The observation of color changes of the liquid crystal material may also be enhanced by applying the material on a substrate, such as a black-pigmented matrix or lattice, or on black polymer film, as disclosed in French Pat. No. 2,110,505 and Canadian Pat. No. 912,806, respectively. Where a liquid crystal tape is used, the tape may include a non-liquid crystal dye of a color adapted to improve the selective reflection of a particular color range, e.g. as taught by German Offenlegungsschrift Nos. 2,018,028 and 2,059,789 or by Canadian Pat. No. 901,277. It is also possible to compound a cream consisting of microencapuslated liquid crystals, carbon black and a surfactant in a water soluble organic solvent such as polyvinyl alcohol, as shown by Canadian Pat. No. 899,610. Such a cream will dry to a tough film and can be peeled off after use.
The encapsulation of the liquid crystal material may be carried out in a number of different ways. For example, small drops of the liquid crystal material may be dispersed in an aqueous mixture of gelatin and gum arabic to form a colloid. The coating that is formed when this mixture is applied to a surface such as the skin, can be hardened by the use of a diol such as perterediol, or by a dialdehyde, e.g. formaldehyde. Various ways of carrying out the latter technique are disclosed in U.S. Pat. Nos. 3,697,297; 3,732,119; 3,585,381; and 3,578,844. However, the encapsulation of liquid crystals is not limited to the use of colloids. For example, a plastic such as polyurethane may be used to form the capsules. Each capsule so formed then comprises a plastic sheath containing a plurality of liquid crystals, e.g. as disclosed in Japanese Pat. Nos. 44,177 (1973) and 71,377 (1973).
As previously mentioned, the mechanism that produces the apparent color of the substance is selective reflection, whereby light of a particular color is reflected back strongly while other colors are substantially absorbed. This effect is due to the ordering of planes of the liquid crystal molecules and the spacing between these planes, i.e. the repeat distance between planes related to the same color. The extent by which the spacing of the repeat distance varies for a change of color covering the entire spectrum, i.e. from red through yellow to blue, may be no more than a factor of 2. Thus, only a small temperature change is required to produce the aforesaid color change if the liquid crystal material is properly selected. For example, cholesteryl oleyl carbonate is capable of traversing the entire color spectrum, i.e. for red to blue, in response to a temperature change of less than 1.degree. C.
The use of liquid crystal materials of the kind mentioned above, particularly where the human body is concerned, has been largely limited in the past to thermographic applications, as discussed earlier. The present invention makes use of these materials to monitor the acoustic intensity of ultrasonic energy applied to a body for the purpose of diagnosing an internal target therein. As such it is capable of providing an indication of an equipment malfunction, which indication can be readily recognized even by a person lacking in technical training.